The present invention relates generally to rolling metal plate, strip and foil, and particularly to a tooling surface finishing process that provides both the work rolls and backup rolls in each stand of a rolling operation with micro-engineered surface textures and coating material. The surface textures and coating material act to minimize the generation of wear debris, provide sufficient rolling traction and control frictional forces by ensuring adequate lubrication in the tooling/workpiece interface. The tooling surface finishing process begins with those rolls used in a reversing (breakdown) mill, that prepares an ingot for further hot and cold rolling by reducing its thickness, and ends with those rolls used in the last stand of a cold rolling mill or foil mill.
With reference to traction, a certain minimum traction is necessary between the work rolls of a mill and plate, strip or foil being rolled to achieve desired reductions in the thickness of the rolled material (i.e., the plate, strip or foil). Traction between back-up rolls and work rolls and between work rolls and the rolled material is problematic since, with liquid lubrication and highly polished rolls, slipping tends to occur between the back-up rolls and the work rolls as well as between the work rolls and rolled material. If the surfaces of ground and chrome plated rolls are rough to the extent that substantial traction is assured, then the rolls will tend to micro-machine the surface of the material being rolled by dislodging material from the rolled surface. This machined material is comprised of metal and hard metal oxide particles which collect on the surface of the rolled material. This is a particularly tenacious problem with aluminum plate, strip or foil since the relatively hard aluminum oxide particles machined from the surface of the rolled material embed themselves into the softer (nascent or un-oxidized) aluminum which can then adhere or back-extrude into the texture of the roll surface producing a roll coating effect. This leads to a continuation of the re-transfer process of hard particles from the roll to the rolled material surface.
The previously mentioned problems attain an even greater significance in a tandem rolling mill with conventionally ground rolls. The individual stands in a tandem rolling mill generally use a single coolant-lubricant which coolant-lubricant is filtered at a single lubricant cleaning location, often referred to as an "oil house." With the generation of excessive quantities of wear debris, the filtering demands on the oil house are substantially increased.
An additional aspect of the tandem mill is the differing lubrication conditions which exist within each stand. These differences in lubrication conditions between individual stands arise from the differing rolls speeds, temperatures, reduction ratios, material thicknesses and front/back tensions applied to the rolled material in each stand. The difference in lubrication conditions are not caused by the use of different coolant/lubricant systems. Rather, the physical and chemical properties of the coolant/lubricant are the same across the mill; one way to change the lubrication conditions in each stand is to optimize the texture of the roll surfaces for that stand.
Difficulties in rolling aluminum arise in part because of the tendency of nascent aluminum to adhere to the roll surfaces. Adhesion is especially pronounced when there is an inadequate amount of lubricant in the roll bite, the quality of the lubricant is poor and the roll surface roughness is such that it contributes to the adhesion problem by ploughing the worked material surface and retaining a small portion of the material. Since the aluminum surface itself is soft, adhesion to the roll surface with subsequent re-transfer of the adhered material to the surface of the rolled material results in severe surface damage to the rolled material in the form of rolled-in dirt and/or black surface streaks which are a mixture of aluminum wear debris and lubricant residue. The rolled material will, therefore, have little market value and must be relegated to scrap, with subsequent recycling. The roll surfaces must be refinished to remove adhered material in order to ensure the surface quality of rolled material in future operations.
In the operation of continuous hot mills and reversing (breakdown) mills, proper lubrication conditions are very difficult to achieve and excessive wear debris transfer to the work roll surface is the rule due to high roll surface and rolled material surface temperatures. The extreme temperatures cause partial evaporation of the lubricant prior to its entering the roll bite. A resulting vapor layer prevents oil "plate-out" on the roll surface. This promotes decomposition of organic lubricant inside the roll bite and subsequently leads to rupture of protecting boundary films on the roll surface. Excessive wear debris transfer is also due to lower rolling speeds, higher roll surface roughness and the substantial thickness reductions of the rolled material. When combined, these factors preclude a hydrodynamic effect and lead to elevated interface pressures, temperatures, high micromachining of the strip surface and an extension of the length of contact between the roll and rolled material in the roll bite. Auxiliary equipment, such as scratch brushes, must be employed to remove metal transferred to the roll surfaces. Failure to remove the transferred material results in substantial metal re-transfer to the rolled material surface, as discussed above. Additional steps, such as the use of a scratch brush, contribute to the mechanical intensity (i.e. another process step and apparatus) of the hot rolling operation, and increase rolling costs.
Severe roll wear between work rolls and backup rolls due to three body abrasion and localized plastic deformation occurs at the location where the largest wear particles preside. This requires redressing of the rolls surfaces which leads to costly down time and an excessive inventory of ground rolls.
All of the previously discussed problems pertaining to the rolling operation may be attributed to current roll shop finishing operations, which generally involve one or more grinding operations. Since grinding is a stochastic process which produces a random surface roughness on the rolls, a roll surface will be unable to properly mitigate lubricant rheology and friction in the roll bite.